1. Field of the Invention
The present invention relates to a stator for a dynamo-electric machine.
2. Description of Related Art
In one type of electric motor (dynamo-electric machine), such as a brushless motor, a rotating magnetic field is generated when electric current is sequentially supplied to coils wound around iron core portions, respectively, of a stator. A rotor is rotated through interaction between the thus generated rotating magnetic field and a magnetic filed generated by magnets or coils of the rotor. In a case of an inner rotor type electric motor, the iron core portions of the stator are arranged radially outward of the rotor, and the coils are wound around the iron core portions, each of which serves as a winding core of the corresponding coil.
In the above stator, core sheets, each of which is made of a thin magnetic plate material, such as a silicon steel plate, are axially stacked one after the other. Such a stator includes a pillar like inner core and a cylindrical outer core. The inner core includes iron core portions and bridges. The iron core portions are arranged radially outward of the rotor, and each bridge connects between radially inner ends of corresponding two of iron core portions. The outer rotor connects radially outer ends of the iron core portions of the inner core together. Each of the inner core and the outer core is formed by stacking the corresponding core sheets, and thereafter the inner core and the outer core are connected together to form the stator.
When the stator is formed by connecting the two separate cores together, the coils can be easily wound around the radially outwardly extending iron core portions upon formation of the inner core through the stacking of the inner core sheets. Thus, the winding operation of the coils to the iron core portions is facilitated. After the winding operation of the coils, the outer core, which is formed by stacking the outer core sheets, is secured to the inner core, to which the coils are wound, and thereby the stator is formed.
In the thus constructed stator, the iron core portions are connected by the bridges. Thus, a portion of a magnetic flux, which is generated by the magnets of the rotor, passes through the bridges and thereby is leaked through the bridges. The leaked magnetic flux does not contribute to rotation of the rotor, so that the leaked magnetic flux has negative influences on a reduction in a size of the motor and an increase in an output power of the motor.
In order to compensate the above disadvantages, Japanese Unexamined Patent Publication No. 9-19089 discloses a stator, in which an axial thickness of each bridge is reduced in comparison to an axial thickness of the rest of the core sheet to reduce an effective cross sectional area of a magnetic passage that conducts a magnetic flux in the bridge. In this way, a magnetic resistance of the magnetic passage is increased, and the leakage of the magnetic flux is limited.
With this arrangement, the magnetic flux is less likely leaked through the bridges, and thereby non-effective magnetic flux, which does not contribute to the rotation of the motor, can be advantageously reduced. Thus, it is possible to increase the output power of the motor and to reduce the size of the motor.
However, in the above motor, although the leakage of the magnetic flux is reduced to increase the output power of the motor by reducing the axial thickness of each bridge, cogging torque (oscillating torque) of the motor is disadvantageously increased due to the reduced axial thickness of each bridge. The cogging torque disadvantageously causes generation of noises and vibrations in the motor.